Cathodic sputtering is a deposition process involving ion bombardment of a target composed of a material to be deposited onto a substrate. The target forms part of a cathode assembly in an evacuated chamber containing an inert gas, such as argon. An electric field supplied between the cathode assembly and an anode in the chamber, and the gas is ionized by collision with electrons ejected from the surface of the cathode, forming a plasma between the target surface and the substrate. The positive gas ions are attracted to the cathode surface, and particles of material dislodged when the ions strike the target then traverse the enclosure and deposit as a thin film onto a substrate or substrates positioned on a support maintained at or near anode potential.
This deposition technique has been applied extensively in electronics for the coating of semiconductor silicon wafers with aluminum or an aluminum alloy in the manufacture of integrated circuits. The manufacture of integrated circuits with a high level of integration, for example, DRAM memories with a capacity greater than 4 MB, requires the deposition of metallic interconnection layers of small thickness (approximately 1 .mu.m), which are then etched to form extremely fine lines (less than 0.5 .mu.m in width) allowing individual access to each memory position. Under these conditions, any defect in the metallization layer, whose size is close to the width of an interconnection line, can lead to a material defect during the etching operation of the integrated circuit, and lead to the rejection of the integrated circuit.
In the case of current and future generations of ultra-large-scale integrated circuits, for example, the DRAM memories of 16 MB or more, the fineness of etching has been considerably stressed, and the width of the line has been brought to a few tenths of a micron, on the order of 0.2-0.5 .mu.m. Defects, such as inclusions, from the target deposited on the semiconductor substrate during cathodic sputtering have become a major cause for rejection of integrated circuits, and each year these defects cost the worldwide electronics industry considerable sums of money.
To reduce the number of thin film coated substrates rejected because of etching defects due to inclusions or other defects in the sputter target, aluminum and aluminum alloy cathode sputter targets are inspected nondestructively for such internal defects. Ultrasonic test methods are typically used for this inspection in which the target is immersed in a liquid and the material is scanned for defects. Typically, the defect size is determined by comparing the amplitude of the signal generated with the signal from an artificial defect of known size machined into a reference target blank. As described in U.S. Pat. No. 5,887,481, an ultrasound sensor or probe is calibrated with respect to artificial defects consisting of flat-bottomed holes having a diameter of 0.1 mm machined into a target composed of an identical alloy with metallurgical characteristics similar to those of the product to be tested. The targets to be tested are then immersed in liquid and the amplitude of the ultrasound echo obtained is compared with the amplitude of the artificial defect to determine the relative size of the defects. Also, the number of echoes exceeding the amplitude corresponding to the artificial defect of 0.1 mm may be counted. By this method, targets having too many defects per unit volume or having defects of large size can be thrown out to prevent the use of such targets which can have a negative effect on sputtered film quality.
A disadvantage of this method is that the maximum allowable defect size in sputter targets that must meet current line width sizes is approximately a factor of 10 smaller than the smallest artificial defect sizes that can feasibly be machined into the reference target blanks. Thus, there is likely to be a great disparity between the size of the actual defects and the size of the artificial defect to which it is being compared.
There is thus a need to develop a method for determining actual defect sizes in cathode sputtering targets subjected to ultrasonic inspection that is accurate and relatively easy to carry out.